Laser-based and LED-based video projectors have been used extensively in business environments and have recently come into wide use in large-screen projection systems in home theaters. The miniaturization of projection systems has led to the development of “pico-projectors” that may be embedded in other systems, such as mobile phones and heads-up displays for vehicle dashboards, or may be implemented as stand-alone devices, such as pocket or ultra-mobile projectors that maybe be powered from a battery or an external power source.
One example of a pico-projector system is the PicoP™ projector engine developed by Microvision, Inc. The PicoP engine includes RGB laser sources, a micro-electro-mechanical system (MEMS) scanning mirror, optics and video processing electronics for receiving video data from a source and generating an image to be projected onto any viewing surface (e.g., a screen, a wall, a sheet of paper or a chair back). However, pico-projection systems such as this that use a MEMS scanning mirror face a number of technical problems that are not as critical in larger projection systems.
A MEMS scanning mirror implemented in a pico-projection system is a two-dimensional scanning mirror, or two separate MEMS mirrors, that sweeps laser beams across a viewing surface similar to the vertical and horizontal sweep of an electron beam in a CRT-based television or monitor. The horizontal sweep is typically done at one of the resonant mode frequencies of the scanning mirror that is on the order of 18 kHz or higher. The vertical sweep is generally desired to be an ideal saw tooth to provide a linear sweep movement from top-to-bottom with minimal retrace time, thus maximizing the allowable active video time.
The two leading methods for generating the horizontal drive signal for a MEMS scanner are phase-locked loop (either digital or analog) and amplitude response peak search. When either of these methods is implemented, rapid and large changes in the brightness of the lasers projected onto the MEMS scanner result in very rapid temperature change of the mirror and/or surrounding structure. This heating causes a shift in the resonant frequency of the MEMS. The frequency change may be small, in the 2 Hz-to-8 Hz range. However, the MEMS resonant mode has a very high Q, which means that a very small change will have a large impact on the MEMS deflection angle. This may result in a large reduction in image size, until the phase-locked loop or peak search loop locks onto the new resonant frequency.